Creation of phase shift photomasks (PSMs) is an example process that relies on good overlay between layers. PSMs are often used to improve the image quality of the exposure of an image reproduced by a stepper on a wafer. There exist a number of conventional methods to create PSMs. But, typically all of these methods require sufficiently accurate alignment between the first and subsequent layers.
Production of Half Tone (HT) masks is another example where sufficiently accurate overlay between layers is important. In HT photomask production, a mask consisting of areas with different transmission regions (e.g., about 0%, about 50% and about 100%) is manufactured. Conventionally, this process involves patterning a workpiece at least twice with intermediate process steps. These types of masks are normally used in order to combine two or more patterns to create a TFT array backplane, which reduces the number of physical masks needed for a certain design.
Manufacturing 3 dimensional (3D) structures, typically used for imprinting stamps or other manufacturing technologies that rely on 3D structures, are also created by subsequent patterning steps with intermediate process steps (e.g., development and/or etching) between the patterning steps. In these manufacturing processes, sufficiently accurate overlay between patterning or definition steps is important to ensure structure quality.
The above-mentioned workpieces, masks, substrates, templates, etc. are normally used in the manufacturing process of display applications such as thin-film transistor-liquid crystal displays (TFT-LCD), organic light emitting diodes (OLEDs), surface-conduction electron-emitter display (SED), plasma display panels (PDPs), field emission displays (FEDs), low temperature poly-silicon LCDs (LTPS-LCD) and similar display technologies. Other areas of use are in the manufacturing of semiconductor devices and support structures such as memories (e.g., SRAM, DRAM, FLASH, ferroelectric, ferromagnetic, etc.), integrated circuits (ICs), printed circuit boards (PCBs), IC substrates, charge coupled device (CCD) sensors, complimentary metal oxide semiconductor (CMOS) sensors, holograms, PWB, etc.
Layer-to-layer overlay is also a somewhat critical property in direct patterning or “maskless” patterning techniques. In these techniques, the template or photomask is normally replaced with a fast pattern generator. The fast pattern generators for maskless production (referred to as “direct write tools”) have the ability to overlay subsequent patterns on the same workpiece; that is directly on the intended device.
Conventionally, second and subsequent layer patterning normally relies on point alignment. When using point alignment a finite number of alignment marks patterned on a reference layer act as a guide for subsequent patterning. Hence, a first layer to be patterned includes alignment marks and after development, etch, resist spinning or other processes the workpiece is ready for the next layer exposure.
Prior to the patterning subsequent layers of the workpiece the alignment marks are read by the patterning system. In order to be able to fit the subsequent layer on top or in a specific relation to the first layer, the system calculates transformation factors (e.g., rotation, translation, scale, orthogonality, etc.). These calculated transformation factors are used in the subsequent patterning step to achieve adequate overlay between the patterned layers.
In theory, by calculating and using transformation factors (e.g., translation, rotation, scaling and/or orthogonality) sufficiently accurate overlay may be achieved. But, this is only true when the workpiece is considered to be essentially rigid.
Unfortunately, in the manufacturing of photomasks, for example, the masks themselves cannot be considered rigid bodies. Instead, the plates suffer from deformation or distortion during the first or subsequent patterning steps. This deformation may be caused by particles on the backside of a workpiece, changes in patterning conditions, pattern design, placement on a plate support, etc. Because these deformations normally lead to local and geometrically complex deformations or distortions, they cannot be compensated for by using simple global parameters.
Conventionally, additional alignment marks may be added in the first or reference layer to achieve a better approximation of how the workpiece distorts between patterning steps. However, the introduction of additional alignment marks is typically impossible because the main part of a workpiece is covered with a functional pattern and therefore cannot include alignment marks.
Furthermore, it is relatively difficult (if not impossible) to achieve an acceptable prediction in advance on where the distortion will take place, and therefore, it is not a viable solution to increase the number of alignment marks in specific areas. Typically, alignment marks are only allowed around the pattern area or image field and are often positioned relatively close to the peripheral edges of the workpiece.
U.S. Pat. No. 7,148,971 introduces the possibility to use Z-correction in a writer and a measurement machine. The Z-correction enables calibration of the reference coordinate system independent of the Z-shape of the reference layer. There exists such a measurement system on the market, but not all makers of large photomasks have acquired such a machine. Thus, it is not a generally accepted standard yet.
The current trend on the market is that large photomasks will get more and more advanced, and for TFT photomasks second layer writing will become more common. In the future, it is likely that masks will become even more advanced, including more than two layers. For more advanced masks, the overlay between layers is of even more importance to achieve a good yield of the final product because usually the plates that use more than one layer writing are the most critical layers in a mask set.
Another growing field of technology which requires sufficiently accurate overlay between written passes is three-dimensional (3D) writing using photolithographic methods. Multi-pass writing may be used to create a 3D structure by performing process steps in-between each exposure to provide the 3D shape. This requires relatively good alignment between each written pass.